1. Field of the Invention
The present invention relates to a method for continuously forming a large area functional deposit film by resolving and exciting material gas with plasma reaction caused by the microwave plasma which is generated evenly over a large area and an apparatus therefor.
More particularly, the invention relates to a method capable of continuously forming a functional deposit film over a large area uniformaly at high speeds with a significant enhancement of the utilization efficiency of the aforesaid material gas and an apparatus therefor, thereby making possible the implementation of a large scale fabrication of large area thin film semiconductor devices such as photovoltaic elements at low cost specifically.
2. Related Background Art
In recent years, electric power consumption has been rapidly increasing worldwide, and along with the active generation of electric power to meet such an enormous demand, the problem of environmental pollution is becoming more serious.
In this regard, in the nuclear power generation which is anticipated as a power generation to take the place of thermal power generation and which has already come into the stage of practical use, there has occurred a serious situation as represented by the accident in Chernobyl Atomic Power Station where serious radioactive contamination have caused damage to the human body as well as have polluting natural environments, leading to the slow down of the future promotion of the nuclear power generation. In fact, there are even some nations that have enacted laws banning the new construction of atomic power stations.
Also, in thermal power generation, the consumption of fossil fuel represented by coal and petroleum has just been on the increase to meet the growing demands on the power, and along with this, the amount of discharged carbon dioxide has increased causing the greenhouse effect gas density of carbon dioxide in the atmosphere to increase, introducing the phenomenon which causes the surface temperature of the earth to be increased. As a matter of fact, the annual average of the earth's temperature is steadily on the increase year after year, and IEA (International Energy Agency) has proposed that the discharge of carbon dioxide should be reduced by 20% by the year 2005.
Whereas such a background as this exists, it is inevitable that the demands on the power will increase in the developing countries along with the increasing populations there. Then, together with the more popularized use of electronics in the life style in the developed nations, resulting in the increased power consumption per capita, the subject of the power supply has reached the stage where its solution should be discussed globally.
Under such circumstances, the power generation by the solar cells which utilize the solar rays of light is now the subject attracting attention as a clean power generation system which can be employed to meet the increasing demand on the power in the future without creating environmental destruction not only because this system does not present such problem as the aforesaid radioactive contamination or temperature rise of the earth surface, but also because there is almost no uneven distribution of energy source thanks to the fact that the solar rays of light cover all over the earth and further a comparatively high generation efficiency without the necessity of complicated large facilities. Various researches and developments of this type of power generation are in progress for its practical use. Now, in order to implement the power generation system using the solar cells to meet the demands on the power, it is fundamentally necessary that the solar cells which are adopted for the purpose should be sufficiently high in the photoelectric conversion efficiency and excellent in the characteristic stability thereof, and that those solar cells can be fabricated in a large scale production.
In this regard, it should be required to provide a solar cell capable of outputting approximately 3 kW per household is order to supply the power generally needed by a family. Then, assuming that the photoelectric efficiency of such a solar cell is approximately 10%, for example, the required area of the solar cell for the needed output should become approximately 30 m.sup.2. Then, to supply the 100,000 households with the power, for example, it is necessary to provide the solar cells having an area of as much as 3,000,000 m.sup.2.
In this respect, there have been proposed various manufacturing methods for fabricating solar cells in a large scale by using a material gas such as gaseous silane which is easily obtainable and by resolving it with glow discharging so as to deposit a semiconductor thin film such as amorphous silicon on a comparatively low cost substrate such as glass or metal sheet in consideration of the possibility of a low-cost production of the solar cells as compared with the solar cells using single crystal silicon and others.
In the power generation using solar cells, it is often adopted to connect the unit modules in series or parallel to unify them for obtaining a desired current and voltage. In this respect, it is a prerequisite that there occur no disconnection or short circuit between each of the modules. It is equally important that there is no irregularity in the output voltage and output current among the modules. Consequently, at least in the stage where the unit modules are fabricated, it is a prerequisite to secure the characteristic uniformity of semiconductor layer itself which is the most significant element to determine its characteristics. Then, from the viewpoint of easier module designing and simpler module assembling process, it is required to provide a semiconductor deposit film having the characteristic uniformity over its large area for enhancing the capability of producing solar cells in a large scale as well as for achieving a significant reduction of manufacturing cost.
Regarding solar cells, the semiconductor layer which is its important component is of the so-called pin junction, and other semiconductor junctions. These semiconductor junctions are implemented by stacking semiconductor layers of different conductivity sequentially or by implanting dopant of different conductivity into a conductive semiconductor layer by the ion implantation method or performing diffusion by thermal diffusion.
If these aspects are observed from the standpoint of the solar cells using the semiconductor thin film of the aforesaid amorphous silicon and others which attract the attention of those who are in the art, the semiconductor film having a desired type of conductivity can be obtained by mixing material gas containing the element which acts as dopant such as phosphine (PH.sub.3), diboran (B.sub.2 H.sub.6), and the like with the silane and others which are main material gas and resolving the mixture by glow discharging in the process of forming such semiconductor films, which are subsequently stacked on a desired substrate sequentially to form deposit layers to implement the semiconductor junction easily. Then, based on this, there has been proposed a method for fabricating amorphous silicon solar cells in such a manner that individual film-formation chambers are provided for forming each of the semiconductor layers and the respective formation of the semiconductor layers is performed in each individual chamber.
In this respect, a continuous plasma CVD apparatus adopting Roll to Roll method is disclosed in the patent specification of U.S. Pat. No. 4,400,409. According to this apparatus, a plurality of glow discharging areas are provided and a sufficiently long plastic substrate having a desired width arranged along the path whereby the aforesaid substrate can be conveyed to penetrate each of the aforesaid glow discharging areas. In each of the glow discharging areas, while the deposit formation of the semiconductor layer of the required type of conductivity is being performed, the aforesaid substrate is continuously conveyed in its longitudinal direction to form the element having the semiconductor junction in succession. Here, in the aforesaid specification, gas gates are employed to prevent the dopant gas used for each of the semiconductor layers from being diffused into the other glow discharging areas so that the gases are mixed. More specifically, the aforesaid glow discharging areas themselves are separated from each other by slit type separating passages and in addition, a means for forming a flow of cleaning gas such as Ar, H.sub.2, or the like is employed in each of the separating passage. With this in view, the Roll to Roll method can be regarded as a method suited for fabricating semiconductors in a large scale. Nevertheless, the aforesaid formation of each semiconductor layer being performed by the plasma CVD using RF (radio frequency), there is automatically a limit in implementing the improvement of the film deposition speed while maintaining the characteristics of the film to be formed continuously. In other words, it is necessary to generate a specific plasma for a considerable length and large area at all times even for forming a semiconductor layer of just 5,000 .ANG. thick as well as to maintain the aforesaid plasma uniformly. However, in order to fulfill such requirements, a considerable displine is needed, making it difficult to generalize various plasma parameters related thereto. Also, the resolving efficiency and utilization efficiency of the material gas for the film formation are rather low, resulting in one of the reasons to make the manufacturing costs high.
Also, in this respect, a deposit film formation apparatus using an improved Roll to Roll method is disclosed in Japanese Patent Laid-Open Application No. 61-288074. In this apparatus, a hood type slacking portion is formed in a part of flexible continuous sheet substrate to be accommodated in its reaction container, and an active seed produced in an activated space which is different from the aforesaid reaction container and some other material gas as required are introduced into the container. Then, this apparatus is characterized by causing the gases to be chemically interacted by thermal energy to form the deposit film on the inner plane of the sheet substrate having the aforesaid hood type slacking portion formed thereon. Thus, by making the deposit film on the inner plane of the hood type slacking portion thereof, it is possible to make the apparatus compact. Furthermore, as the active seed which has been activated in advance is used, it is possible to make the film formation speed faster than the conventional deposit film formation apparatus.
Nevertheless, this apparatus utilizes the deposit film formation reaction by the chemical interaction with the presence of thermal energy, and in order to make a further improvement of the film formation speed, it is necessary to introduce more amount of the active seed and more supply of the thermal energy. There are automatically limits in the method of supplying a large amount of thermal energy uniformly and in the method of generating a large amount of active seed having a high reactivity to be introduced in a reaction space without loss.
Meanwhile, a plasma process using microwave has attracted the attention of those who are in the art recently. Since the microwave has a shorter frequency band, it is possible to intensify the energy density as compared with the conventional method using the RF, and this process is considered more suitably adaptable to generating plasma more efficiently.
For example, in the specifications of U.S. Pat. No. 4,517,223 and U.S. Pat. No. 4,504,518, a method of forming thin deposit film on a small area substrate in a low-pressure microwave glow discharging plasma is disclosed. According to this method, it is not only possible to obtain a high-quality deposit film because the polimerization of active seed which can be a cause of film characteristic deterioration is prevented because of the low-pressure process, but also possible to restrain the generation of particles of polysilane and others in plasma as well as to implement a substantial improvement of the deposition speed. However, there is no specific disclosure as to the performance of any uniform deposit film formation in a large area.
Meanwhile, in the specification of U.S. Pat. No. 4,729,341, there are disclosed a low-pressure microwave plasma CVD method and apparatus therefor to form a photoconductive semiconductor deposit thin film on a large area cylindrical substrate by using a pair of radiator type waveguide applicators for a high-power processing. However, the large area substrate is limited only to a cylindrical substrate, that is, a drum used as a photoreceptor for electronic photography, and there is no disclosure as to the application to the large area and long substrate. Also, the manufacturing process of the deposit film is of a butch system and the quantity of the deposit film that can be fabricated per charge is limited. There is no disclosure as to the method for forming deposit film in a large quantity continuously on a large area substrate, either.
Now, the plasma using microwave has the short wavelength which is apt to generate the unevenness of energy. Therefore, there are various problems still to be solved as to the application of the plasma using microwave to the large area processing.
For example, a slow-wave circuit can be utilized as an effective means to make the microwave energy uniform, but this slow-wave circuit has its particular problem that a rapid drop in microwave coupling to plasma occurs as the distance increases in the lateral direction of the microwave applicator. Therefore, as a means of solving this problem, a method is attempted to make the energy density uniform in the vicinity of the surface of the substrate where the distance between the processing object and the slow-wave circuit should be changed. Such a method is disclosed in the specifications of U.S. Pat. No. 3,814,983 and U.S. Pat. No. 4,521,717, for example. In the former, it is disclosed that while the slow-wave circuit should be inclined at an angle to the substrate, the transmitting efficiency of the microwave energy to the plasma is not satisfactory. Also, in the latter, while it is disclosed that two non-parallel slow-wave circuits should be provided within the area parallel to the substrate. In other words, the planes themselves which are perpendicular to the center of the microwave applicator should preferably be arranged within the surface parallel to the object substrate in such a manner that these planes intersect each other on the straight line at right angles to the conveying direction of the substrate. Also, it is disclosed that in order to avoid the interference between the two applicators, the applicators themselves should be arranged with a deviation laterally in the conveying direction of the substrate in a length corresponding to half the cross bar of the waveguide. Also, several proposals are made as to methods of maintaining the uniformity of the plasma (i.e., the uniformity of energy). Such proposals are noticed, for example, in the reports published in Journal of Vacuum Science Technology, B-4 (January - February, 1986) pp. 295-298 and in the same journal B-4 (January - February, 1986) pp. 126-130. According to these reports, a microwave reactor called microwave plasma-disc-source (MPDS) has been proposed. In other words, the plasma is of a disc type or table type and its diameter is the function of microwave frequency. Then, the reports disclose the contents such as given below. In other words, at first, the plasma-disc-source can be varied by the microwave frequency. However, in a microwave-plasma disc-source designed to be operated by 2.45 GHz, the confinement diameter of plasma is as small as approximately 10 cm and the plasma volume is also just 118 cm.sup.3. This cannot be regards as the area having been enlarged at all. Also, according to the aforesaid reports, in a system designed to be operated at a low frequency as 915 MHz, it is possible to obtain the plasma diameter of approximately 40 cm and the plasma volume of 2,000 cm.sup.3 by lowering the frequency. The aforesaid reports disclose further that by operating at a lower frequency, 400 MHz for example, the discharge can be expanded to a diameter of more than 1 m. However, the apparatus which can attain this objective requires an extremely expensive specific system.
In other words, it is possible to attain making the plasma area large by lowering the microwave frequency. However, the microwave source capable of providing a high output in such a low frequency band is not available in general and it is difficult to procure the source or even if it can be procured, only at an extremely high cost. Also, then, it is more difficult to procure a high output variable frequency microwave source.
Likewise, there are proposed in Japanese Patent Laid-Open Application No. 55-141729 and Japanese Patent Laid-Open Application No. 57-133636 a method as means for generating a high density plasma efficiently by the use of microwave, in which the ECR (electron cyclotron resonance) condition is satisfied by arranging electromagnets around a cavity resonator, and in its academic society and others, many reports have been made to disclose the formation of various semiconductor thin films by the utilization of this high density plasma. A microwave ECR plasma CVD apparatus of the kind has already been available commercially on the market.
However, in the methods using the ECR, the magnets are adopted to control the plasma. Thus, in addition to the ununiformity of the plasma due to the wavelength of the microwave, there occurs the ununiformity in the magnetic field distribution, making it technically difficult to form the deposit film evenly over the large area substrate. Also, in a case where an apparatus should be made larger to deal with a large area, the electromagnets become great automatically, hence creating many problems to be solved before the system is put into practice, such as the weight and space which will be increased accompanying the use of the larger magnets, the necessity of countermeasure against the heat generation, and the provision of the stabilized direct current source for supplying a large current.
Further, as regards the deposit film thus formed, the characteristics thereof have not reached the equal level as the one formed by the conventional RF plasma CVD method, and the deposit film formed in the space where the ECR condition is satisfied is extremely different from the deposit film formed in the space other than ECR condition, such as the space of the so-called divergent magnetic field in the characteristics and deposition speeds. As a result, this method cannot be regarded as a suitable method for fabricating the semiconductor device which requires especially a high-quality and uniformity.
In the aforesaid specifications of the U.S. Pat. No. 4,517,223 and U.S. Pat. No. 4,729,341, it is disclosed that the maintenance of extremely low pressures is required in order to obtain the high density plasma. In other words, it is absolutely necessary to perform the processing under low pressures for the enhancement of the gas utilization efficiency. However, in order to maintain the relationship between the high deposition speed, high gas utilization efficiency, high power density, and the low pressure, the application of either the slow-wave circuit or the electron cyclone resonance method disclosed in the aforesaid patent specification is still insufficient.
Therefore, it is desired that a new microwave plasma process should be provided as early as possible by solving the various problems existing in the microwave means mentioned above.
Now, the thin film semiconductors are suitably used not only for the fabrication of the aforesaid solar cells but also for the thin film transistors (TFT) for driving the pixels of a liquid crystal display, photoelectric conversion element for the contact image sensors, switching elements and the like, for which it is imperative that the thin semiconductor device to be used should have a large area and a long length. While some of them have already in practical use as a key component for the aforesaid image input/output apparatus, for example, it is expected that the provision of a new deposit film formation method whereby to fabricate a high-quality large area film having an excellent uniformity at high speeds enables the thin film semiconductor to be used more widely in general.
Also, in the microwave plasma CVD method, when a large current microwave is introduced for the deposit film formation to resolve the film formation gas, it has generally been a practice as a method to propagate the microwaves generated by a microwave source through a waveguide and introduce them into a film formation chamber through a dielectric having an excellent microwave transmissivity (hereinafter referred to as microwave transmittable member) for the resolution of the film formation gas.
According to this method, however, the plasma of the film formation gas using the microwave is in contact with the surface of the aforesaid microwave transmittable member and therefore the problem is created that the deposit film also adheres to the surface of the microwave transmittable member.
The deposit film adhering to the surface of the microwave transmittable member becomes a reflecting element or absorbing element of microwaves although its degree differs depending on the material to be deposited thereon, and causes the microwaves passing through the microwave transmittable member to be reduced, creating the problem that such a deposition results in slowing down the deposition speed of the deposit film or changing the characteristics of the deposit film.
Also, the deposit film adhering to the microwave transmittable member absorbs the microwaves to cause it to be heated and temperature difference is generated in the microwave transmitting member, leading to the possibility that this member will eventually be damaged.
When the deposit film should be formed continuously for a long time or the microwave introducing current is large so that the deposit film formation speed is high, the above-mentioned problem is more serious because the film thickness of the deposit film adhering to the microwave transmittable member becomes great.
Therefore, it is necessary to replace the aforesaid microwave transmitting member or to perform an etching of the deposit film adhering to the microwave transmittable member before a trouble such as the reduction of the microwave transmittance or the damage on the microwave transmitting member may result.
The frequency with which the microwave transmitting member should be replaced depends greatly on the kinds of the deposit film to be formed or the deposition speed of the deposit film adhering to the microwave transmitting member. However, while the microwave transmittable members are being replaced, the formation of the deposit film should be suspended. Consequently, the longer it takes to replace the microwave transmitting members, the more the production speed of the deposit film is reduced. For example, if the film deposition chamber should be leaked for the replacement of the microwave transmittable members, then the air in the film deposition chamber is again exhausted and a time is required until the influence of the water absorbed is reduced. Also, there is a method for performing a dry etching in the film deposition chamber to remove the deposit film which has adhered to the microwave transmittable member, but the composition of the dry etching gas remains in the film deposition chamber, thus causing in some cases the characteristics of the deposit film to be lowered because of the mixture of this remaining composition with the deposit film when its formation is resumed.